In exhaust systems of internal combustion engines which are operated with a lean fuel/air mixture, that is to say with a mixture with a lambda value >1, what is referred to as a lean NOx trap may be present in the exhaust duct in order to absorb nitrogen oxides (NOx) and therefore improve the emission values of the engine. The lean NOx trap typically has a plurality of ducts through which the engine exhaust gases are conducted and at the surface of which the nitrogen oxides are bound chemically with the aid of, for example, alkali metals such as, for example, barium or strontium. In the case of barium, for example, the nitrogen oxides are bound in the form of barium nitrate.
Since the storage capacity of the lean NOx trap is finite, it is regenerated from time to time, wherein the stored nitrogen oxides, therefore the barium nitrates in the above example, are converted into nitrogen dioxide (NO2) which is subsequently converted into innocuous nitrogen molecules (N2), carbon dioxide (CO2), and water (H2O) using noble metals in the lean NOx trap. This regeneration of the lean NOx trap is carried out by means of hydrocarbons and/or carbon monoxide with which the trap is purged. The hydrocarbons or the carbon monoxide are made available as a result of the fact that the internal combustion engine is temporarily operated with a rich fuel/air mixture (lambda<1). A method for purging a lean NOx trap is described, for example, in DE 102 49 017 A1.
An advantage of the lean NOx traps is, in particular, that the nitrogen oxides can be absorbed from the exhaust gas even during cold starting so that the emissions of the engine are reduced even during cold starting. However, a situation may occur in which the capacity of the lean NOx trap is exhausted when cold starting occurs. This may lead to an increase in nitrogen oxide emissions of the engine since further absorption of the nitrogen oxides by the lean NOx trap may not be possible before purging has taken place, which in turn firstly requires a specific minimum temperature in the trap to be reached. Due to the initial cold engine temperature, the time to reach the minimum temperature may be prolonged, resulting in the release of excess NOx to the atmosphere.
The inventors have herein recognized the above issues and provide a method to at least partly address them. In one embodiment, a method for operating an internal combustion engine having a lean NOx trap connected downstream comprises, in a normal operating mode, operating the internal combustion engine with a lean fuel/air mixture, and in a special operating mode, operating the internal combustion engine with a rich fuel/air mixture in order to bring about regeneration of the lean NOx trap, wherein a changeover from the normal operating mode to the special operating mode takes place when switching off of the internal combustion engine is expected.
In the method according to the disclosure for operating an internal combustion engine having a lean NOx trap connected downstream, the internal combustion engine is operated with a fuel/air mixture. In this context, in a normal operating mode the internal combustion engine is operated with a lean fuel/air mixture (lambda>1), and in a special operating mode it is operated with a rich fuel/air mixture (lambda<1). The special operating mode serves to bring about regeneration of the lean NOx trap, which is also referred to as purging of the lean NOx trap. According to the disclosure, a changeover from the normal operating mode, that is to say the operating mode of the internal combustion engine with the lean fuel/air mixture (lambda>1), to the special operating mode, that is to say the operating mode of the internal combustion engine with the rich fuel/air mixture (lambda<1), takes place when switching off of the internal combustion engine is to be expected.
Since, according to the disclosure, purging of the lean NOx trap takes place whenever switching off of the internal combustion engine is imminent or at least there is a certain probability that it is imminent, it is possible to ensure that in the case of cold starting of the engine a purged lean NOx trap which can absorb the nitrogen oxides occurring during cold starting is available. The method according to the disclosure therefore has a technical result of lowering the nitrogen oxide emissions of the internal combustion engine during cold starting by ensuring that the lean NOx trap is in a purged state during cold starting.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.